Dual frequency feed

Abstract

 A duel frequency feed for use with an antenna permits reception of signals in both a low frequency band and a high frequency band and is com­prised of a single feed through which both signals are propagated, a high frequency probe which extends concentrically within the throat of the feed for receiving the high frequency signal, a wave guide connected to the throat of the feed including a pair of turnstile junctions with four wave guides inter­connecting the turnstile junctions, and a single output port with a polarization rotation device for receiving the low frequency signal. The four inter­connecting wave guides may be either co-axial cables or generally rectangular wave guides. The rectangu­lar wave guide version is made from a number of cast aluminum parts and has flanges for assembly, and both versions are characterized by propagating low fre­quency signals of different polarizations through the guide to the same exit port so that a single low fre­quency pick-up can be utilized to receive any of the differently polarized low frequency signals. Other embodiments show different connections both at the throat and at the low frequency output.

Description

Background and Summary

[0001] The satellite television reception (TVRO) industry has mushroomed in recent years, and continues to mushroom as more and more people are learning of the vast array of television programming which is accessible to them with the installation of a TVRO earth station. Still other factors are the increas­ing number of products available to a consumer, and the steadily decreasing cost for such a system. A typical system includes as one of its main components an antenna which is used to collect the signals from the various satellites. As is well known in the art, there are a band of satellites in geosynchronous orbit above the equator which broadcasts television signals, and the antenna's job is to collect those signals from the specific satellite to which it is pointed. At the present time, most of these satel­lite television signals are broadcast in C-band, or at a frequency range of 3.7 to 4.2 GHz. However, some of the signals are being broadcast at Ku-band, or frequencies ranging from 11.7 to 12.2 GHz. Be­cause of the many advantages offered by Ku-band, more and more programmers are switching to Ku-band, and satellites being placed in orbit are in ever-­increasing numbers utilizing Ku-band. Some observers even predict that Ku-band will replace C-band en­tirely as the C-band satellites end their useful life and fall out of orbit and are replaced by Ku-band satellites.

[0002] To take full advantage of the programming available from the satellites presently in orbit, there is a real need for the antenna to be capable of receiving signals at both C-band and Ku-band. To complicate matters further, the signals broadcast at each band are of both horizontal and vertical polar­ity, so the feed should be capable of receiving and making available for selection both polarizations. Presently, with the C-band feeds well known in the industry, a single polarization rotation device is usually mounted in the feed, and it includes a probe or other signal pick-up structure which can be ori­ented to select either vertical or horizontal polar­ity. This capability is desirable to quickly change from one signal to another and thereby view the full complement of television signals broadcast by any one particular satellite. This device also makes correc­tion for skew very easy by slightly moving the probe. However, this device requires that signals of both polarization are present in the same exit port.

[0003] The inventors herein are aware of at least two prior art dual frequency feed horns which are shown in U.S. Patent 3,389,394 and U.S. Patent 3,500,419. The '394 patent discloses a multiple fre­quency feed horn which utilizes a common input of a circular wave guide, the walls of which are used to conduct the low frequency signal to a pair of dipole antennas, and which contains a co-axially mounted dielectric horn which is used to receive the high frequency portion of the signal. This structure has a single feed for the high frequency signal, but utilizes two separate dipole antennas and two sepa­rate co-axial connectors and lines to receive the low frequency signal. Similarly, the '419 patent dis­closes a feed with a high frequency probe extending concentrically through the interior of a low fre­quency horn, but the horn has four slot apertures for low frequency signals, a pair of apertures being used for each of the two differently polarized signals. A pair of half height wave guides are attached to each pair of slot apertures and are joined in a Y config­uration to provide a separate feed for each of the two polarized low frequency signals. Therefore, for the feeds of either of these prior art structures, the polarization rotation device which is presently widely used cannot be utilized, and instead separate low frequency signal pick ups would be required to pick up the two differently polarized signals broad­cast at low frequency. Still another more serious problem with these two prior art feeds is that there is no easy way to adjust for skew. With the polari­zation rotation devices pesently available, skew can be easily adjusted by merely rotating the signal pick-up structure. Instead, with the construction of these prior art feeds, the entire feed would have to be rotated.

[0004] To solve these and other problems, the inventors herein have succeeded in developing a dual frequency feed which includes a high frequency probe concentrically mounted within a low frequency feed horn, which is highly desirable as it eliminates the problems and complications with offset feeds, and which also incorporates a wave guide attached to the throat of the low frequency feed for conducting low frequency signals of both polarizations such that a polarization rotation device presently available can be mounted to the wave guide and used to select be­tween low frequency signals of different polariza­tions. In a first embodiment, the wave guide achieves this by utilizing a first turnstile junction mounted adjacent the throat of the low frequency feed which branches into four substantially rectangular, off axis wave guides extending parallel to the central axis of the feed. These wave guides and the low fre­quency signals conducted through them are then re­combined in a second turnstile junction which is co-axial with the low frequency feed, high frequency probe, and first turnstile junction, and which exits through a single circular wave guide and a pair of step transitions into a single polarization rotation device. To facilitate the mounting and stability of the high frequency probe, a collar is provided on the first turnstile junction through which the probe is inserted, the diameter of the collar and probe being matched to provide an engagement therebetween to stabilize the probe in its proper orientation.

[0005] The wave guide including the two turnstile junctions and the substantially circular input and output sections can be integrally formed by a plural­ity of cast aluminum pieces, with flanges formed along the edges of the cast aluminum pieces to facil­itate bolting of the pieces together around the high frequency probe. A tuning element may be provided consisting of an upstanding rod axially located in the second turnstile junction to reject the unwanted low frequency modes and direct the waves into the exit guide. The step transitions at the exit portion of the guide permit the higher order modes to die out before reception by the probe of the low frequency polarization rotation device. Also, a mode ring is fitted to the mouth of the throat of the wave guide to improve the illumination pattern of the feed, as is well known in the art.

[0006] Still another feature of the present inven­tion is the construction of the high frequency probe. Generally, the high frequency probe may be a hollow metal cylinder, such as aluminum. However, to adapt the high frequency probe for use with the same re­flector as is utilized for the low frequency band, a dielectric plug is utilized to "spoil" the Ku-band beam and thereby increase the electrical aperture of the probe. This broader beam width substantially de-sensitizes the placement of the Ku-band probe, and helps to minimize the effect on performance from improper installation, or shifting of the position of the feed over time due to weathering, wind loading, or the like. This dielectric insert may be a cast polystyrene plug which is simple inserted within the tip of the probe.

[0007] As mentioned above, the feed of the present invention permits reception of both C-band and Ku-band signals through a single feed where the signals are co-mingled at the horn input, and where the low fre­quency signals of both polarization are propagated through a single wave guide to a single exit port where the low frequency signal of either polarization may be detected or picked up with the presently known polarization rotation device. This is achieved with a Ku-band probe and C-band feed which are co-axially aligned for optimum utilization of the reflector and antenna.

[0008] In a second embodiment of the present in­vention, the off-axis rectangular wave guides may be eliminated and replaced by co-axial cables with probes extending into the square portion of circular-­to-square transition, thereby forming cable turn­stile junctions, mounted both at the throat of the feed and at the transition to the low frequency polarization rotation device. These co-axial cables have probes for receiving the signal within the cable turnstile and launching the signal at the other end. Care must be taken to maintain the length of the co-­axial cables so that there is no phase imbalance or power mismatch at the output cable turnstile. How­ever, if manufactured properly, this embodiment does provide some cost savings over the cast aluminum off-­axis rectangular wave guides of the first embodiment.

[0009] In a third embodiment of the present inven­tion, the co-axial cables are utilized, but their associated probes are inserted through the outer mode ring of the feed, and not into a cable turnstile junction connected to the throat. With the Ku-band probe inserted through the inner throat of the feed, the inner throat acts as a reciprocal dummy to excite the proper mode within the mode ring, as desired. Thus, the high frequency probe receives and detects the high frequency signal, while the four low fre­quency probes mounted to the outer ring receive the low frequency signal. As C-band transmission is in both vertical and horizontal polarization, the four low frequency probes are best positioned symmetrically about the circular mode ring, with the top and bottom probes thus receiving vertically polarized signals, and the right and left probes receiving horizontally polarized signals. These separately detected signals are then re-combined in a cable turnstile junction within which a second set of probes are mounted at the other ends of the co-axial cables. This embodi­ment may not achieve the same gain as is thought to be attainable in some of the other embodiments of the present invention, but it does benefit from a further anticipated cost reduction by eliminating the first cable turnstile as is used in the second embodiment of this invention.

[0010] In a fourth embodiment of the invention, an orthomode junction (which is essentially a turnstile junction having two of its outputs shorted) is con­nected through a circular-to-square transition to the throat of the feed, and the Ku-band probe band is inserted through the back of the orthomode junction and concentrically within the throat of the feed as in the other embodiments. This embodiment does pro­vide co-mingling of both high frequency and low fre­quency signals at the throat of the feed, but requires two separate low frequency pick-up means at its out­put to detect and receive both polarizations of the low frequency signal. Thus, this embodiment does not provide the inherent advantage offered by the other embodiments of this invention in that two low fre­quency signal pick-up means must be used, but it does offer a simpler design and anticipated lower cost to construct than some of the other embodiments. Fur­thermore, this embodiment also requires rotation of the feed to adjust for skew, although its simpler construction, and anticipated lighter weight does alleviate this problem somewhat. In a broad sense, the orthomode junction which is used to terminate the wave guide, is in the same family as the turnstile junctions utilized in the other embodiment. Hence, when the term "turnstile junction" is used herein, it is meant to refer to any of these constructions.

[0011] In the foregoing description and explana­tion of the present invention, it has been assumed that its major application has been to the TVRO industry, and, in particular, as a feed means with an antenna having a main reflector. However, this need not necessarily be the case as the feed itself can and does function as an antenna for low gain applica­tions. This can include applications wherein data is transmitted through spread spectrum technology. Fur­thermore, the frequencies mentioned herein are C-band and Ku-band. However, it is anticipated that these bands may themselves be replaced in coming years such that still higher frequency bands are utilized there­by making the feed of the present invention more suit­able for direct use as an antenna by itself. Thus, the inventors herein anticipate that this invention has applications well beyond the specific embodiments and applications disclosed herein.

[0012] The foregoing has been a brief description of some of the principal advantages and features of the present invention which may be more fully under­stood by referring to the drawings and description of the preferred embodiment which follows.

Brief Description of the Drawings

[0013] 

Figure 1 is a side view of a typical prime focus TVRO antenna with the improved feed means of the present invention mounted at the focal point thereof;

Figure 2 is a front view of the improved feed means taken along the plane of line 2-2 in Figure 1;

Figure 3 is a back view of the improved feed means taken along the plane of line 3-3 in Figure 1;

Figure 4 is a cross-sectional view of the improved feed means taken along the plane of line 4-4 in Figure 3;

Figure 5 is a cross-sectional view of the throat of the wave guide taken along the plane of line 5-5 in Figure 4;

Figure 6 is a cross-sectional view of the four substantially rectangular wave guides extending between the two turnstile junctions taken along the plane of line 6-6 in Figure 4;

Figure 7 is a cross-sectional view of the rear of the wave guide detailing the step transitions and polarization rotation device taken along the plane of line 7-7 in Figure 4;

Figure 8 is an oblique view of the second embodiment of the improved feed means of the present invention utilizing co-axial cables as a portion of the wave guide;

Figure 9 is an enlarged cutaway view de­tailing the probes associated with the co-axial cables of the embodiments shown in Figure 8;

Figure 10 is an oblique view of the third embodiment of the present invention showing direct mounting of the low frequency probes within the outer mode ring;

Figure 11 is an oblique view of the fourth embodiment of the feed means of the present invention showing the use of an orthomode junction; and

Figure 12 is an oblique view of still another embodiment of the present invention showing the use of a corrugated S-shaped profiled horn.

Detailed Description of the Preferred Embodiment

[0014] An antenna 20 as might be used for a TVRO application is shown in Figure 1 and includes a re­flector 22 mounted to a mast 24 by an antenna mount 26 with a linear actuator 28 connected between the reflector 22 and the antenna mount 26 to drive the reflector 22 in the azimuth direction to facilitate pointing of the antenna 20 to any one of the group of satellites in geosynchronous orbit above the equator, as is known in the art. A button hook or mast 30 extends outwardly from the reflector 22 and provides a mounting for a feed 32 of the present invention at the electrical focal point of the reflector 22, as known in the art.

[0015] As best shown in Figure 4, the principal elements of the feed 32 include a mode ring 34 mounted to the throat 36 of a wave guide which is generally designated as 38. A high frequency probe 40 extends co-axially through the throat 36 and mode ring 34, as shown. A dielectric insert 41, which may be made of cast Polystyrene, is inserted into the tip of probe 40, and broadens the probe 40 beam width to facilitate its usage with reflector 22. The wave guide 38 includes a first turnstile junction 42 which branches into four rectangular wave guides 44 and then recombines in a second turnstile junction 46. A tuning element 47 is comprised of a generally cylin­drical, upstanding post which extends into the second turnstile junction 46 and, as known in the art, tunes the junction 46 to reject unwanted modes and direct the signal therethrough. Two step transitions 48, 50 are formed in the circular wave guide exit portion 52, and a polarization rotation device 54 is mounted at the exit port 56, as is known in the art. A for­ward strut 58 and a rear strut 60 mount the feed 32 from mast 30, and a plurality of guy wires 62 may, if necessary, be mounted to the feed 32 and extend to the edge of reflector 22 (as shown in Figure 1) to further stabilize the feed 32 to maintain it in posi­tion.

[0016] The mode ring 34 and throat 36 are shown in greater detail in Figures 2 and 5 wherein the mode ring includes an outer ring 64 and an inner ring 66, with an offset difference in height between them, as is known in the art, to maximize the electrical per­formance thereof. The entire wave guide 38 including the throat 36 may be formed from four cast aluminum members, with flanges 68 and bolts 70 used to assemble the wave guide 38. Also, a plurality of bolts 72 ex­tend through flange 74 to mount the mode ring 34 to throat 36.

[0017] The first turnstile junction 42, rectangu­lar wave guides 44, and high frequency probe 40 are best shown in Figure 6. As shown therein, each wave guide 44 is a full height wave guide and is joined by flanges 74 and bolts 76. The four rectangular wave guides 44 are off-axis but symmetrically spaced about the center axis of the high frequency probe 40. Fur­thermore, a collar 78 is formed at the rear of the turnstile junction 42 and through which probe 40 is mounted to stabilize probe 40 and retain it in posi­tion. As is evident from Figures 4 and 6, the turn­stile junction 42 has a single entry port through circular wave guide throat 36 and four substantially rectangular wave guide branches 44. As is known in the art, with the arrangement shown, low frequency signals of one polarization will split between oppo­site rectangular wave guide branches 44, such as the top and bottom branches, while the other polarization will split between the other two rectangular wave guide branches 44, such as the left and right branches. These split signals will recombine in the second turnstile junction 46 before entry into the wave guide exit portion 52, including step transi­tions 48, 50. This is best shown in Figure 4.

[0018] The polarization rotation device 54 in­cludes a probe 80 which is connected to a motor 82 for rotation thereof as necessary to select the sig­ nal and polarization desired to be received. Also as known in the art, the probe 80 may be slightly moved to adjust for skew. The received signal is launched into the low noise amplifier 84 at the low frequency end, and the high frequency signal is received and the differently polarized signals are separated in the high frequency receiver 86.

[0019] A second embodiment 88 of the present in­vention is shown in Figures 8 and 9 and includes a cable turnstile junction 90 connected to the throat 92, with four co-axial cables 94 extending between transition 90 and a second cable turnstile junction 96. As detailed in Figure 9, each co-axial cable 94 is mounted to an end wall 98 of each of junctions 90, 96, and is terminated in a probe 100 for reception or launching of the low frequency signal. As is known in the art, the vertially oriented probes 100 receive and launch the vertically polarized low frequency signal while the horizontally oriented probes 100 re­ceive and launch the horizontally polarized low fre­quency signal. This second embodiment 88 thus elimi­nates the cast aluminum wave guide 38 of the first embodiment and replaces it with the co-axial cables 94 and cable turnstiles 90, 96.

[0020] A third embodiment 102 is shown in Figure 10 and includes an inner throat 104 and an outer throat 106, with four co-axial cables 108 terminating in probes 110 through the outer throat 106 to pick up the low frequency signal therein. The high frequency probe 112 extends through the inner throat 104 such that the inner throat 104 acts as a reciprocal dummy wherein there is little, if any, low frequency signal propagated. A cable turnstile junction 114 receives the other ends of the co-axial cables 108, and re­combines the low frequency signals for propagation to a low frequency pick-up means (not shown). This embodiment 102 differs in operation from the first two embodiments in that the low frequency signal is only propagated in the outer throat, while the high frequency signal is only propagated in the inner throat.

[0021] A fourth embodiment 116 utilizes an ortho­mode junction 118 as the terminating structure for the wave guide 120 comprised of a circular-to-square transition 122 connected to throat 124. This embodi­ment 116 differs from the previous embodiments in that a separate low frequency signal pick-up means (not shown) must be connected to each of the two out­put ports 126, 128 for detection of a singly polar­ized low frequency signal. For example, the ortho­mode junction 118 would propagate a vertically polar­ized low frequency signal through output port 126 and a horizontally polarized low frequency signal through output port 128 if installed as shown in Figure 11. The high frequency probe 130 is inserted through the back of orthomode junction 118 and extends generally concentrically within throat 124, as shown.

[0022] Still another embodiment 132 is shown in Figure 12 and includes generally the same structure as shown in the first embodiment, except that a cor­rugated profiled S-shaped horn 134 is used to detect the low frequency signal, horn 134 providing somewhat greater gain than the feeds used in the other embodi­ments herein. Thus, embodiment 132 might be more suitably used directly as an antenna immediately for low gain applications such as spread spectrum data transmission and reception. However, the other embodiments shown herein might be equally utilized.

[0023] There are various changes and modifications which may be made to applicant's invention as would be apparent to those skilled in the art. However, these changes or modifications are included in the teaching of applicants' disclosure, and it is in­tended that the invention be limited only by the scope of the claims appended hereto.

Claims

1. An antenna for at least receiving signals broadcast from one of a group of satellites, said antenna having a main reflector dish, said dish hav­ing a pre-determined shape to focus the signals from said satellite at a desired location, said satellites having means to broadcast signals in either a low frequency range or a high frequency range, the anten­na being characterized by an improved feed means hav­ing a single throat in which signals of both fre­quency ranges are co-mingled, a mode ring secured to the throat and extending outwardly therefrom, a wave guide means through which the low frequency signal is propagated comprising a first turnstile junction con­nected to the throat, a second turnstile junction, and a plurality of wave guides extending between said first and second turnstile junctions, a low frequency signal pick-up means connected to the second turn­stile junction, and a high frequency probe extending through the first turnstile junction, throat, and mode ring, and being generally concentric therewith.

2. The device of Claim 1 further comprising a collar extending outwardly from the first turnstile junction, the high frequency probe extending there­through, the collar and probe being dimensioned to provide mechanical support to the probe.

3. The device of Claim 2 further comprising a tuning element in the second turnstile junction, and at least one step transition mounted between the sec­ond turnstile junction and the low frequency signal pick-up means.

4. The device of Claim 1 wherein the plurality of wave guides comprise four rigid pipes, each of said pipes being generally rectangular in cross-section.

5. The device of Claim 4 wherein the plurality of wave guides comprise a plurality of cast members, said cast members having flanges to facilitate their assembly, and wherein the high frequency probe, throat, first and second turnstile junctions, and the low frequency signal pick-up means are all generally co-axial.

6. The device of Claim 1 wherein the plurality of wave guides comprise a plurality of co-axial cables.

7. The device of Claim 1 further comprising means to alter the beam width of the high frequency probe.

8. The device of Claim 7 wherein said high frequency beam width control comprises a dielectric plug inserted within the tip of the high frequency probe.

9. The device of Claim 1 wherein the low fre­quency signal pick-up means has means to selectively pick up low frequency signals of different polarities.

10. An antenna for at least receiving signals broadcast from one of a group of satellites, said antenna having a main reflector dish, said dish hav­ing a pre-determined shape to focus the signals from said satellite at a desired location, said satellites having means to broadcast signals in either a low frequency range or a high frequency range, the anten­na being characterized by an improved feed means for receiving signals of both frequencies, the feed means including an inner throat, an outer throat, said throats being generally concentric, a plurality of low frequency probes extending into said outer throat for receiving the low frequency signals, a wave guide means, a low frequency signal pick-up means, said wave guide means interconnecting said low frequency probes and the low frequency signal pick-up means, and a high frequency probe extending generally con­centrically into the inner throat.

11. The device of claim 10 wherein the wave guide means comprises a plurality of cables, each one of said cables having an end connected to an associ­ated low frequency probe, and junction means joining the other ends of said cables, said junction means having means to combine the low frequency signals being propagated through the cables, said junction means having a single output port for connection to the low frequency probe so that all of the low fre­quency signals received by said feed means are propa­gated to a single low frequency signal pick-up means.

12. The device of Claim 11 wherein low fre­quency signals of two polarizations are broadcast, said junction means comprises a turnstile junction, and the plurality of low frequency probes comprises at least two probes, each probe being adapted to re­ceive low frequency signals of only one polarization.

13. The device of Claim 12 wherein the plural­ity of low frequency probes comprises four probes, two of the probes being adapted to receive low fre­quency signals of one polarization, and the other two probes being adapted to receive low frequency signals of the other polarization.

14. An antenna for at least receiving signals broadcast from one of a group of satellites, said antenna having a main reflector dish, said dish hav­ing a pre-determined shape to focus the signals from said satellite at a desired location, said satellites having means to broadcast signals in either a low frequency range or a high frequency range, the anten­na being characterized by an improved feed means hav­ing a single throat, a wave guide means through which the low frequency signal is propagated, an orthomode junction connected to the wave guide means, a high frequency probe, means for mounting the high fre­quency probe through the orthomode junction and con­centrically in the throat, and low frequency signal pick-up means connected to the output of the ortho­mode junction.

15. The device of claim 14 wherein the wave guide means includes a circular-to-square transition, the orthomode junction being connected to the square end of said transition, and wherein the orthomode junction has two outputs, said junction having means to separately propagate low frequency signals of one polarization through one of said outputs and low fre­quency signals of another polarization through the other of said outputs, and further comprising a sepa­rate low frequency pick up connected to each of said outputs.

Drawing